My Series 2 has started to over-rev itself when you put your foot down without going any great speed (Will rev to 3500rpm in 3rd, 4th gear at 20mph! )

When you put your foot down it revs out like you're riding the clutch, but doen't have any grunt!

First started last Saturday when I was driving down a short carriageway near home & suddenly revved like I'd selected 1st gear when I had infact changed into 3rd, then I couldn't get it in any gear & had to coast to a layby & after a couple of minutes it released & I could select all gears again!

Could it be the clutch slipping after performing an accidental burnout in the rain on Friday afternoon in the rain?

Any ideas?